Problems associated with the generation and discharge of electrostatic charge during the manufacture and use of photographic film and paper products have been recognized for many years by the photographic industry. The accumulation of charge on film or paper surfaces can cause difficulties in support conveyance, as well as lead to attraction of dust, which can produce fog, desensitization, repellency spots and other physical defects. The discharge of accumulated static charge during or after the application of sensitized emulsion layer(s) can produce irregular fog patterns or "static marks". The severity of static problems has been exacerbated greatly by increases in sensitivity of new emulsions, coating machine speeds, and post-coating drying efficiency. The charge generated during the coating process results primarily from the tendency of high dielectric constant polymeric film base webs to undergo triboelectric charging during winding and unwinding operations, during transport through coating machines, and during post-coating operations such as slitting, perforating and spooling. Static charge can also be generated during the use of the finished photographic product. In an automatic camera, the repeated winding and unwinding of the photographic film in and out of the film cassette can result in generation of electrostatic charge, especially in a low relative humidity environment. The accumulation of charge on the film surface results in the attraction and adhesion of dust to the film and can even produce static marking. Similarly, high-speed automated film processing equipment can produce static that produces marking. Sheet films are especially subject to static charging during use in automated high-speed film cassette loaders (e.g., x-ray films, graphic arts films, etc.)
In order to eliminate problems arising from electrostatic charging, there are various well known methods by which an electrically-conductive antistatic layer can be introduced into the photographic element to dissipate accumulated static charge, for example, as a subbing layer, an intermediate layer, as an outermost layer overlying a silver halide emulsion layer, as a backing layer on the opposite side of the support from the silver halide emulsion layer(s) or on both sides of the support. A wide variety of conductive antistatic agents can be used in antistatic layers to produce a broad range of electrical conductivities. Many of the traditional antistatic layers used in photographic elements employ materials which exhibit predominantly ionic conductivity. Antistatic layers containing simple inorganic salts, alkali metal salts of surfactants, alkali metal ion-stabilized colloidal metal oxide sols, ionic conductive polymers or polymeric electrolytes containing alkali metal salts and the like have been taught in prior art. The electrical conductivities of such ionic conductors are typically strongly dependent on the temperature and relative humidity of the surrounding environment. At low relative humidities and temperatures, the diffusional mobilities of the charge carrying ions are greatly reduced and the bulk conductivity is substantially decreased. Further, at high relative humidities, an unprotected antistatic backing layer containing such an ionic conducting material can absorb water, swell, and soften. Especially in the case of roll films, this can result in the adhesion (viz., ferrotyping) and even physical transfer of portions of a backing layer to a surface layer on the emulsion side of the film (viz., blocking).
Antistatic layers containing electronic conductors such as conjugated conductive polymers, conductive carbon particles, crystalline semiconductor particles, amorphous semiconductive fibrils, and continuous semiconductive thin films or networks can be used more effectively than ionic conductors to dissipate charge because their electrical conductivity is independent of relative humidity and only slightly influenced by ambient temperature. Of the various types of electronic conductors disclosed in prior art, electronically-conductive metal-containing particles, such as semiconductive metal oxides, are particularly effective when dispersed with suitable polymeric binders. Binary metal oxides doped with appropriate donor heteroatoms or containing oxygen deficiencies have been disclosed in prior art to be useful in antistatic layers for photographic elements, for example: U.S. Pat. Nos. 4,275,103; 4,416,963; 4,495,276; 4,394,441; 4,418,141; 4,431,764; 4,495,276; 4,571,361; 4,999,276; 5,122,445; 5,294,525; 5,382,494; 5,459,021; and others. Suitable claimed conductive metal oxides include: zinc oxide, titania, tin oxide, alumina, indium oxide, silica, magnesia, zirconia, barium oxide, molybdenum trioxide, tungsten trioxide, and vanadium pentoxide. Preferred doped conductive metal oxide granular particles include Sb-doped tin oxide, Al-doped zinc oxide, and Nb-doped titania. Additional preferred conductive ternary metal oxides disclosed in U.S. Pat. No. 5,368,995 include zinc antimonate and indium antimonate. Other suitable electrically-conductive metal-containing granular particles including metal borides, carbides, nitrides, and silicides have been disclosed in Japanese Kokai No. 04-055,492.
Antistatic backing or subbing layers containing colloidal "amorphous" vanadium pentoxide, especially silver-doped vanadium pentoxide, are described in U.S. Pat. Nos. 4,203,769 and 5,439,785 and others. Colloidal vanadium pentoxide is composed of entangled microscopic fibrils or ribbons 0.005-0.01 .mu.m wide, about 0.001 .mu.m thick, and 0.1-1 .mu.m in length. However, colloidal vanadium pentoxide is soluble at the high pH typical of developer solutions for wet photographic processing and must be protected by a nonpermeable, barrier layer. Examples of suitable barrier layers are taught in U.S. Pat. Nos. 5,006,451; 5,221,598; 5,284,714; and 5,366,855, for example.
In order to improve the durability of the antistatic layer and adhesion to underlying or overlying layers it is generally preferred to disperse the colloidal vanadium pentoxide in a polymeric film-forming binder. However, due to the solution chemistry and oxidative potential of vanadium oxide, the selection of compatible binders is limited. For example, for low coating coverages the vanadium pentoxide may typically be coated at 0.05 wt. % or less. At such low concentrations the vanadium pentoxide is prone to instability and flocculation. Depolymerization of vanadium pentoxide gel may also occur at low concentrations or low pH values. A film-forming sulfopolyester latex or polyesterionomer binder can be combined with colloidal vanadium pentoxide in the conductive layer for improved solution stability and to minimize degradation during processing as taught in U.S. Pat. Nos. 5,360,706; 5,380,584; 5,427,835; 5,576,163; and others.
U.S. Pat. No. 5,439,785 teaches the use of a specified ratio of sulfopolymer to vanadium oxide to provide an antistatic formulation which remains conductive after photographic processing. A weight ratio range of from 1:20 to 1:150 V.sub.2 O.sub.5 :sulfopolymer is specified. Surface electrical resistivity values are typically greater than 1.times.10.sup.9 ohm/square for the indicated range. At lower colloidal vanadium oxide concentrations, the conductivity is insufficient to provide antistatic protection; at higher vanadium oxide concentrations the antistatic layer loses conductivity when subjected to photographic processing. However, prior art colloidal vanadium pentoxide typically have significantly lower resistivity values, i.e., 1.times.10.sup.8 ohm/square. Consequently, one of the primary benefits of colloidal vanadium oxide, low resistivity at low dry weight coverage is not achieved.
U.S. Pat. No. 5,718,995 teaches an antistatic layer containing colloidal vanadium oxide and a specified polyurethane binder having excellent adhesion to surface treated polyester supports and an overlying transparent magnetic layer. However, it is further disclosed that the coating composition has limited shelf-life (less then 48 hrs). In order to overcome the limited shelf life, a mixed melt process was preferably used in which separate solutions of colloidal vanadium pentoxide and of the polyurethane binder were prepared and mixed in-line just prior to the coating hopper. This results in an undesirable complication of the coating process. It is further disclosed in '995 that it is difficult to achieve adequate adhesion to glow discharge treated polyethylene naphthalate for a magnetics backing package consisting of a solvent coated cellulosic-based magnetic layer overlying an antistatic layer containing colloidal vanadium pentoxide and the preferred sulfopolyesters or interpolymers of vinylidene chloride cited in the above mentioned U.S. Patents.
In addition to the aqueous-based coating compositions described above it may be advantageous to coat antistatic layers from solvent-based formulations. U.S. Pat. No. 5,709,984 describes antistatic layers containing colloidal vanadium oxide gel, a volatile aromatic compound, and a polymeric binder prepared from a solvent-based dispersion using acetone and ethanol. Polymeric binders demonstrated include interpolymers of vinylidene chloride, polymethylmethacrylate, cellulose nitrate and cellulose diacetate. It is further disclosed that due to the exceptional adhesion requirements of antistatic layers containing colloidal vanadium oxide, such layers generally exhibit poor adhesion when directly coated on untreated or unsubbed polyester supports.
U.S. Pat. No. 5,356,468 teaches the use of cellulose nitrate as a binder or co-binder which imparts improved solution stability for solvent based coating formulations. The addition of cellulose nitrate to a formulation of vanadium oxide gel in a solvent mixture of acetone, alcohol and water resulted in improved resistance to precipitation when exposed to cellulose triacetate film supports.
U.S. Pat. No. 5,366,544 teaches the use of cellulose acetate having an acetyl content of from 15 to 35 weight percent as a binder for vanadium pentoxide. It is further disclosed to use a solvent mixture consisting of dialkyl ketone, an alkanol, and water.
U.S. Pat. No. 5,455,153 describes photographic elements containing a clad vanadium pentoxide layer. The cladding layer is formed by applying an overcoat of an oxidatively polymerizable compound which may be applied neat to the vanadium oxide or in the form of an aqueous solution, a solvent solution or as a vapor. Suitable oxidatively polymerizable monomers include anilines, pyrroles, thiophenes, furans, selenophenes and tellurophenes. Antistatic layers containing clad vanadium oxide were demonstrated to have improved resistance to basic solutions as typically encountered during conventional photographic processing. Improved base resistance results from cladding the surface of vanadium pentoxide rather than a change resulting from polymer intercalation between vanadium oxide layers.
Intercalation of various species, including cations, metal-containing complexes, organic molecules and polymers, within vanadium oxide is well-known, particularly in the catalysis field and as cathode materials for batteries. However, intercalated colloidal vanadium oxide for antistatic applications has not typically been addressed.
U.S. Pat. No. 5,659,034 describes intercalation of metal coordination complexes, particularly Zn(2,2'-dipyridyl).sub.2, between layers of vanadium oxide. The resultant intercalated vanadium oxide was described as black rod-shaped crystals which are unsuitable for antistatic applications for photographic films.
U.S. Pat. No. 5,073,360 describes the formation of bridged/lamellar metallic oxides having intercalated spheroidal cationic species. The preferred metallic oxide is vanadium pentoxide and the spheroidal cationic species is preferably an aluminum polyoxocation, particularly [Al.sub.13 O.sub.4 (OH).sub.24 ].sub.7.sup.+. The vanadium oxide gel can be prepared for example by ion exchange or melt quenching. The intercalated material is then isolated by filtration, dried and optionally calcined to give high surface area materials which are particularly suited as molecular sieve filters, catalysts, and catalyst supports. However, no indication is given regarding the antistatic properties of the intercalated vanadium oxide.
Intercalation of a wide variety of organic or polymeric materials between vanadium oxide layers in vanadium oxide gels is well known. Intercalative polymerization of aniline resulting in polyaniline is described in Mater. Res. Soc. Symp. Proc. V. 233, pp. 183-194, 1991 and Chem. Mater. V. 8, pp. 1992-2004, 1996. A significant decrease in oxygen concentration and a color change from red to dark blue was observed when vanadium oxide gel was added to an air saturated solution of aniline in water. Conductivity of the polyaniline-vanadium oxide material increased substantially upon aging. It was proposed that conductivity in the fresh material occurred by electron transport through the vanadium oxide framework (semiconductive) but upon aging a metallic-like conductivity dominated as polyaniline chains formed.
Poly(ethylene oxide) intercalated vanadium oxide gels were reported in Chem. Mater, Vol. 3, 992-994, 1991 and Chem. Mater, Vol. 8, 525-534, 1996 to be highly light sensitive, turning dark blue within several weeks for exposure to room light or within several hours for exposure to UV irradiation. Non-intercalated vanadium oxide gels were not light sensitive. In addition to a color change, the conductivity increased and solubility decreased with increasing irradiation. However, the irradiated conductivity decreased with increasing polyethylene oxide intercalation. Changes in the vanadium oxide interlayer distance due to intercalation of poly(vinylpyrrolidone) (PVP), poly(propyleneglycol) (PPG), and methylcellulose are described in Adv. Mater, Vol. 5, 369-372, 1993. Interlayer distance increased linearly for (PVP).sub.x V.sub.2 O.sub.5.nH.sub.2 O for values of x up to 3. Furthermore, a change in the chemical nature of PVP was noted and ascribed to formation of hydrogen bonding with co-intercalated water. The interlayer spacing did not vary linearly with either PPG or methylcellulose. The interlayer distance remained constant for (PPG).sub.x V.sub.2 O.sub.5.nH.sub.2 O with x values greater than 1, and PPG remained chemically unaltered. Particularly in the case of PPG, the samples were light sensitive as indicated above.
The above references indicate a vast array of organic or polymeric species can be intercalated within vanadium oxide gel structures. However, the intercalated material is frequently light sensitive and conductivity changes during aging. Furthermore, intercalation and subsequent reaction frequently decreases solubility of the vanadium oxide gel.
The use of polyvinylpyrrolidone in antistatic formulations is also well known. For example, U.S. Pat. Nos. 4,418,141; 4,495,276; 5,368,995; 5,484,694; 5,453,350; 5,514,528 and others include polyvinylpyrrolidone amongst an extensive list of suitable binders for antistatic materials such as tin oxide or zinc antimonate. There is no specific mention or claim to enhanced properties or stability of polyvinylpyrrolidone or other water soluble vinyl-containing polymers relative to other polymeric binders for the above mentioned patents.
U.S. Pat. No. 4,489,152 describes a diffusion transfer film having an opaque layer consisting of carbon black having 2-10 percent polyvinylpyrrolidone based on the weight of carbon black. The addition of polyvinylpyrrolidone having a molecular weight of about 10,000 to the carbon black layer was found to improve the silver transfer process. However, there was no indication of antistatic properties nor of formulation stability for the carbon black layer.
U.S. Pat. No. 4,860,754 describes an electrically conductive adhesive material consisting of a low molecular weight plasticizer, a high molecular weight water soluble, crosslinkable polymer, uncrosslinked polyvinylpyrrolidone, and an electrolyte. The uncrosslinked polyvinylpyrrolidone is added as a tackifier. Antistatic properties are insufficient for photographic applications since the electrolyte can be removed during wet photographic processing. Furthermore, ionic conductors are generally not effective when overcoated with a hyrdrophobic layer such as a transparent magnetic recording layer.
U.S. Pat. No. 5,637,368 describes the use of colloidal dispersions of vanadium oxide for imparting antistatic properties to adhesive tapes. Polyvinylpyrrolidone and polyvinylpyrrolidone copolymers are included in a list of suitable adhesive compounds. The use of vanadium oxide in the adhesive layer is suggested, but all examples consist of a separate vanadium oxide layer and a separate adhesive layer. In addition polyvinylpyrrolidone was not demonstrated nor disclosed to give superior performance. Furthermore, use of the adhesive material having antistatic properties for use in photographic imaging applications is not suggested.
As disclosed in the above mentioned U.S. Patents several polymers, for example interpolymers of vinylidene chloride, sulfopolyesters, polyesterionomers, and cellulosics have been used as binders for antistatic layers containing colloidal vanadium oxide. However, due to the solution chemistry and oxidative potential of vanadium oxide, the selection of compatible binders and formulation range is limited. For example, for low coating coverages the vanadium pentoxide may typically be coated at 0.05 weight percent or less. Such low concentrations result in coating formulations which are prone to instability and flocculation of the vanadium oxide gel. This creates serious difficulties in accumulation of flocculated vanadium oxide plugging solution delivery lines, filters and coating hoppers. Furthermore, flocculation can result in coating defects or "slugs" which can result in optical and electrical non-uniformities in the coating. The addition of surfactants to the coating solution may stabilize the vanadium oxide gel, however, the typically high levels of surfactant required are undesirable for adhesion and coatability of subsequently applied layers. The concern of stability has been addressed in many of the above patents. Furthermore, interaction between colloidal vanadium oxide and polymeric binders can result in limited dispersion shelf-life. In addition to the potential for incompatibility of binders, it is well known that vanadium pentoxide can act as a reactant or catalyst for decomposition of organic solvents. Decomposition products can adversely impact the coating quality of the antistatic layer and potentially adversely impact the sensitometric performance of photographic emulsions thereby requiring careful selection of coating solvents and binders for the antistatic layer. Furthermore, due to the potential interaction of vanadium pentoxide with solvents and binders, careful consideration must be given to formulation of overlying layers, such as barrier layers and abrasion resistant layers.
Because the requirements for an electrically-conductive layer to be useful in an imaging element are extremely demanding, the art has long sought to develop improved conductive layers exhibiting a balance of the necessary chemical, physical, optical, and electrical properties. As indicated hereinabove, the prior art for providing electrically-conductive layers useful for imaging elements is extensive and a wide variety of suitable electroconductive materials have been disclosed. However, there is still a critical need in the art for improved conductive layers which can be used in a wide variety of imaging elements, which can be manufactured at a reasonable cost, which are resistant to the effects of humidity change, which are durable and abrasion-resistant, which do not exhibit adverse sensitometric or photographic effects, which exhibit acceptable adhesion to overlying or underlying layers, which exhibit suitable cohesion, which have improved solution stability, which have improved binder compatibility, and which have low catalytic or reactant activity. In particular, an improved colloidal vanadium oxide which is compatible with a wider selection of polymeric binders or facilitates the use of higher binder:vanadium oxide ratios to improve adhesion to the support and underlying or overlying layers is desired. It is toward the objective of providing an electrically-conductive layer that more effectively meet the diverse needs of imaging elements, especially those of silver halide photographic films, but also of a wide range of other types of imaging elements, than those of the prior art that the present invention is directed.